sched: cfs core, kernel/sched_idletask.c

add kernel/sched_idletask.c - which implements the idle thread
scheduling class. This further simplifies sched.c (under CFS),
for example a number of 'if (p == rq->idle)' type of special-cases
can be removed from sched.c, and schedule() gets simpler too.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched_idletask.c

+/*
+ * idle-task scheduling class.
+ *
+ * (NOTE: these are not related to SCHED_IDLE tasks which are
+ *  handled in sched_fair.c)
+ */
+
+/*
+ * Idle tasks are unconditionally rescheduled:
+ */
+static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p)
+{
+	resched_task(rq->idle);
+}
+
+static struct task_struct *pick_next_task_idle(struct rq *rq, u64 now)
+{
+	schedstat_inc(rq, sched_goidle);
+
+	return rq->idle;
+}
+
+/*
+ * It is not legal to sleep in the idle task - print a warning
+ * message if some code attempts to do it:
+ */
+static void
+dequeue_task_idle(struct rq *rq, struct task_struct *p, int sleep, u64 now)
+{
+	spin_unlock_irq(&rq->lock);
+	printk(KERN_ERR "bad: scheduling from the idle thread!\n");
+	dump_stack();
+	spin_lock_irq(&rq->lock);
+}
+
+static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, u64 now)
+{
+}
+
+static int
+load_balance_idle(struct rq *this_rq, int this_cpu, struct rq *busiest,
+			unsigned long max_nr_move, unsigned long max_load_move,
+			struct sched_domain *sd, enum cpu_idle_type idle,
+			int *all_pinned, unsigned long *total_load_moved)
+{
+	return 0;
+}
+
+static void task_tick_idle(struct rq *rq, struct task_struct *curr)
+{
+}
+
+/*
+ * Simple, special scheduling class for the per-CPU idle tasks:
+ */
+static struct sched_class idle_sched_class __read_mostly = {
+	/* no enqueue/yield_task for idle tasks */
+
+	/* dequeue is not valid, we print a debug message there: */
+	.dequeue_task		= dequeue_task_idle,
+
+	.check_preempt_curr	= check_preempt_curr_idle,
+
+	.pick_next_task		= pick_next_task_idle,
+	.put_prev_task		= put_prev_task_idle,
+
+	.load_balance		= load_balance_idle,
+
+	.task_tick		= task_tick_idle,
+	/* no .task_new for idle tasks */
+};